The present invention relates to a semiconductor memory device and more particularly to a sense amplifier control circuit capable of stabilizing memory cell voltage characteristics by clamping an external power supply voltage to an internal power supply voltage.
As the area occupied by a single transistor is reduced in proportion to the density of a semiconductor memory device, the size of transistor is miniaturized. If a voltage from an external power supply voltage terminal is directly applied to a small-sized sense amplifier and memory cell, the voltage characteristics of the cells is distorted. Furthermore, upon performance of a sensing operation, peak current is increased and power noise is generated, thus making it impossible to perform stabilized sensing operation of the memory device. This unstable performance is, in part, due to the fact that, in sense amplifier control circuits, the external power supply voltage is directly applied by an active restoring signal to the sense amplifier and memory cell through a sense amplifier driver transistor.
FIG. 1 is a block diagram illustrating a conventional sense amplifier control circuit classified in accordance with each function. In the figure, a dotted block illustrates a sense amplifier control circuit 50, which is comprised of a comparator 50A, a trigger circuit 50D, a level converting circuit 50B, a comparator enable device 50C, a bias circuit 50E, and a p-type sense amplifier driver control circuit 50F.
The comparator 50A inputs and compares a voltage of a p-type sense amplifier 40 of a memory cell array 100 and a reference voltage Vref and outputs the compared voltage by the control of a p-type sense amplifier control circuit enable signal .phi.SP.
The level converting circuit 50B converts a second power supply voltage of the p-type sense amplifier control circuit enable signal .phi.SP into a first power supply voltage. Typically, the first power supply voltage is an external power supply voltage having a potential of 5 V, and the second power supply voltage is an internal power supply voltage having a potential of 4 V.
The comparator enable device 50C enables or disables the output of the comparator 50A through the level converting circuit 50B.
The trigger circuit 50D inputs, inverts and outputs the outputs of the comparator 50A and comparator enable device 50C.
The bias circuit 50E inputs the output of the trigger circuit 50D and ensures that a current flowing into a driving element of the p-type sense amplifier driver control circuit 50F is constant. The p-type sense amplifier driver control circuit 50F inputs the outputs of the trigger circuit 50D and bias circuit 50E, outputs them to a gate terminal of a p-type sense amplifier driver 1, and then makes a flow of current of the p-type sense amplifier driver 1 constant.
FIG. 2 is a detailed circuit diagram illustrating the conventional sense amplifier control circuit of FIG. 1. FIG. 3A is diagram illustrating a circuit for generating a clock signal of FIG. 2. FIG. 3B is a timing diagram illustrating an operation of FIG. 2. FIG. 3C is a detailed timing diagram illustrating an operation of a portion of FIG. 3B.
Referring to FIG. 2, the comparator 50A comprises a first PMOS transistor 11 having a source terminal connected to an external power supply voltage terminal and a gate terminal and a drain terminal are diode-connected to each other, a second PMOS transistor 12 having a source terminal connected to the external power supply voltage terminal and a gate terminal connected to the gate terminal of the first PMOS transistor 11, a first NMOS transistor 13 having a gate terminal connected to a p-type sense amplifier enable node 2 and a drain terminal connected to the drain terminal of the first PMOS transistor 11, a second NMOS transistor 14 having a gate terminal connected to a predetermined reference voltage Vref and a drain terminal connected to the drain terminal of the second PMOS transistor 12, an output node N1 connected to a common terminal between the second PMOS transistor 12 and the second NMOS transistor 14, and a third NMOS transistor 15 having a drain terminal connected to each of source terminals of the first and second NMOS transistors 13 and 14, a gate terminal connected to the p-type sense amplifier control circuit enable signal .phi.SP, and a source terminal connected to a ground potential terminal.
The level converting circuit 50B comprises a first PMOS transistor 19 having a source terminal connected to an external power supply voltage terminal, a second PMOS transistor 20 having a source terminal connected to an external power supply voltage terminal, a first NMOS transistor 21 having a gate terminal connected to the p-type sense amplifier control circuit enable signal .phi.SP, a source terminal connected to a ground potential terminal, and a drain terminal connected commonly to a drain terminal of the first PMOS transistor 19 and a gate terminal of the second PMOS transistor 20, an inverter 23 having an input terminal connected to the p-type sense amplifier control circuit enable signal .phi.SP and having an internal power supply voltage as a control input, a second NMOS transistor 22 having a source terminal connected to a ground potential terminal and a gate terminal connected to an output terminal of the inverter 23, and output node N3 connected commonly to a gate terminal of the first PMOS transistor 19 and each drain terminals of the second PMOS and NMOS transistors 20 and 22.
The comparator enable device 50C is comprised of a PMOS transistor 16 having a source terminal connected to an external power supply voltage terminal, a gate terminal connected to an output node of the level converting circuit 50B, and a drain terminal connected to the output node of the comparator 50A. The trigger circuit 50D includes a PMOS transistor 17 having a source terminal connected to an external power supply voltage and a gate terminal connected to an output node of the comparator 50A, an NMOS transistor 18 having a source terminal connected to a ground potential terminal and a gate terminal connected to the output node of the comparator 50A, and an output node N2 connected commonly to each of the drain terminals of the PMOS and NMOS transistors 17 and 18.
The bias circuit 50E includes a PMOS transistor 24 having a source terminal connected to an internal power supply voltage terminal and a gate terminal connected to a ground potential terminal, a first NMOS transistor 25 having a drain terminal connected to the drain terminal of the PMOS transistor 24 and a gate terminal connected to the output node N2 of the trigger circuit 50D, an output node as a common terminal of the PMOS and the first NMOS transistors 24 and 25, for outputting a voltage VB, and a second NMOS transistor 26 having drain and gate terminals diode-connected to the source terminal of the first NMOS transistor 25 and a source terminal connected to a ground potential terminal.
The p-type sense amplifier driver control circuit 50F is comprised of a PMOS transistor 27 having a source terminal connected to an external power supply voltage terminal and a gate terminal connected to the output node of the trigger circuit 50D, a first NMOS transistor 28 having a gate terminal connected to a first input line, an output line connecting a common terminal of the PMOS and first NMOS transistors 27 and 28 to a control terminal of the sense amplifier driver 1 and generating a p-type sense amplifier driver enable clock .phi.PSE, a second NMOS transistor 29 having a drain terminal and a source terminal connected between the source terminal of the first NMOS transistor 28 and a ground potential terminal, and a gate terminal connected to the output node of the bias circuit 50E, and a device connected between an external power supply voltage terminal and the output line, for allowing a current iB flowing to the first and second NMOS transistors 28 and 29 to be constant. Here, the device is comprised of a first PMOS transistor 30 having a source terminal connected to the external power supply voltage terminal and gate and drain terminals diode-connected to each other, and a second PMOS transistor 31 having both terminals connected between the drain terminal of the first PMOS transistor 30 and the output line, and having a gate terminal connected to a ground potential terminal. Here, the external power supply voltage terminal outputs an external power supply voltage ext. Vcc, and the internal power supply voltage terminal outputs an internal power supply voltage int. Vcc.
A detailed description of an operation of FIG. 2 will be given with reference to FIGS. 3A to 3C.
Referring to FIG. 3A showing circuits which generate each clock signals, an n-type sense amplifier driver enable clock .phi.NSE, as shown in (b) of FIG. 3A, is generated after a signal .phi.S passing through a delay circuit and a master clock .phi.R are passed through a NAND gate 4. The master clock .phi.R is generated by a row address strobe signal RAS that is passed through inverters 1 to 3, as shown in (a) of FIG. 3A. The p-type sense amplifier control circuit enable clock .phi.SP is generated after the n-type sense amplifier driver enable clock .phi.NSE passing through inverters 6 to 8 and the master clock .phi.R passing through an inverter 9 are inputted to a NOR gate 10 as two inputs and then passed through inverters 11 and 12.
In FIG. 2, a p-type sense amplifier enable clock SAP is initially precharged to half of the internal power supply voltage int. Vcc. That is, when the row address strobe signal RAS is at a precharged state of a logic "high" level, the p-type sense amplifier control circuit enable clock .phi.SP becomes at a logic "low" state and the second NMOS transistor 22 is turned ON, whereby the level converting circuit 50B produces an output having the logic "low" state. Thereby, the PMOS transistor 16 of the comparator enable device 50C is turned ON and the output of the comparator 50A is disabled, thus inputting a signal being at the logic "high" state to the trigger circuit 50D. The trigger circuit 50D produces an output having the logic "low" state and the first NMOS transistor 25 is turned OFF, whereby the bias circuit 50E produces an output having the logic "high" state. The p-type sense amplifier driver control circuit 50F produces an output having the logic "high" state and renders the PMOS sense amplifier driver 1 to be turned OFF, whereby the p-type sense amplifier enable signal SAP is precharged to half of the internal power supply voltage int. Vcc.
An explanation of functions and operations of the bias circuit 50E and the p-type sense amplifier driver control circuit 50F will be given in the following.
The bias circuit 50E includes the output line connected to the gate terminal of the second NMOS transistor 29 serving as a driving element of the p-type sense amplifier driver control circuit, thereby making the current iB constant. Namely, Since a voltage at the node N2 increases as the external power supply voltage ext. Vcc increases, the gate-source voltage Vgs of the first NMOS transistor 25 increases to thereby reduce the voltage V. The gate-source voltage Vgs of the second NMOS transistor 29 in the p-type sense amplifier driver circuit 50F decreases, so that the increment of the current iB caused due to the increment of the external power supply voltage ext. Vcc can be prevented. At this time, since the voltage V supplied from the bias circuit 50E is continuously applied to the gate terminal of the second NMOS transistor 29, the second NMOS transistor 29 is not turned OFF at all.
Since the voltage at the node N2 decreases as the external power supply voltage ext. Vcc decreases, the gate-source voltage Vgs of the first NMOS transistor 25 decreases to thereby increase the voltage V.sub.B. The gate-source voltage Vgs of the second NMOS transistor 29 in the driver circuit 50F decreases, so that the decrement of the current iB caused due to the decrement of the external power supply voltage ext. Vcc can be prevented. This results in making a drain-source current I.sub.DS flowing in the PMOS sense amplifier driver 1 constant. Thereby, the p-type sense amplifier enable signal SAP irrespective of the variation of the external power supply voltage ext. Vcc has a rising tilt.
In the p-type sense amplifier driver control circuit 50F, if the node N2 is at a logic "low" state, the p-type sense amplifier driver enable clock .phi.PSE goes to the logic "high" state to thereby turn OFF the p-type sense amplifier drivers 1 and 5. To the contrary, if the node N2 is at a logic "high" state, the p-type sense amplifier driver enable clock .phi.PSE goes to the logic "low" state to thereby turn ON the p-type sense amplifier drivers 1 and 5 and the first NMOS transistor 28, thus rendering the current iB constant. Even though the external power supply voltage ext. Vcc increases, the current iB is constant thus to maintain a constant voltage of the p-type sense amplifier driver enable clock .phi.PSE. Since the drain-source voltage of the p-type sense amplifier drivers 1 and 5 increases as the external power supply voltage ext. Vcc increases, if a current flowing in the channel is to be constant, the voltage of the p-type sense amplifier driver enable clock .phi.PSE as gate voltages of the p-type sense amplifier drivers 1 and 5 increases to enable a gate source voltage Vgs to be decreased, thereby making a current flowing in the p-type sense amplifier drivers 1 and 5 constant. Accordingly, as a channel current irrespective of the external power supply voltage ext. Vcc is supplied, the rising tilt of the p-type sense amplifier enable clock SAP is all constant in both areas of the "low" external power supply voltage ext. Vcc and the "high" external power supply voltage ext. Vcc, whereby the rising tilt of the p-type sense amplifier enable clock SAP is sharply inclined in the area of the "high" external power supply voltage ext. Vcc. As a result, it is appreciated that the increment of the peak current can be prevented.
An operation of FIG. 2 will be explained hereinafter, when the row address strobe signal RAS is at the logic "low" state to thereby designate a row address.
An arbitrary word line is selected by the row address decoding and division of an electric charge is then made between a bit line and a cell. The n-type sense amplifier driver enable clock .phi.NSE is at the logic "high" state to turn ON the NMOS sense amplifier drivers 3 and 7, thus enabling the bit line to be sensed. The p-type sense amplifier control circuit enable clock .phi.SP goes to the logic "high" state and the output node N3 of the level converting circuit 50B reaches the external power supply voltage ext. Vcc to turn OFF the PMOS transistor 16 of the comparator enable device 50C, thereby enabling the output of the comparator 50A. Since the comparator 50A has a reference voltage Vref (4 V) as a higher voltage than that of the p-type sense amplifier enable clock SAP, the comparator 50A enables the output at the node N1 to be at the logic "low" state. Here, the p-type sense amplifier control circuit enable clock .phi.SP is applied to the gate terminal of the third NMOS transistor 15 of the comparator 50A. The trigger circuit 50D detects the node N1 being at the logic "low" state and outputs the node N2 being at the logic "high" state. The output of the trigger circuit 50D being at the logic "high" state enables the bias circuit 50E and the p-type sense amplifier driver control circuit 50F. As previously discussed, as the bias circuit 50E and the driver control circuit 50F operate, the p-type sense amplifier driver enable clock .phi.PSE turns ON the p-type sense amplifier drivers 1 and 5, thereby enabling a voltage of the p-type sense amplifier enable clock SAP to have a constant rising tilt regardless of the change of the external power supply voltage ext. Vcc. Further, if the voltage of the p-type sense amplifier enable clock SAP reaches a potential of 4 V, the gate voltage of the NMOS transistor 13 of the comparator 50A increases, thus enabling the output of the node N1 to be at the logic "high" state, the output of the trigger circuit 50D to be at the logic "low" state, and the output of the bias circuit 50E to become the internal power supply voltage int. Vcc.
Since the first NMOS transistor 28 of the p-type sense amplifier driver control circuit 50F is turned OFF, the p-type sense amplifier driver enable clock .phi.PSE goes to the logic "high" state and the p-type sense amplifier drivers 1 and 5 are turned OFF, thereby prohibiting the voltage of the p-type sense amplifier enable signal SAP from being over 4 V. If the voltage of the p-type sense amplifier enable clock SAP is maintained at a potential of 4 V and then falls below 4 V due to a leakage current of each transistor as a component of the memory cell and sense amplifier, as shown in FIG. 3C, the voltage of the p-type sense amplifier enable signal SAP rises again by the operation of the comparator 50A.
In the conventional sense amplifier control circuit, under the condition that the voltage on the bit line BL is bumped to some extent, the p-type sense amplifier driver 1 of FIG. 2 is slightly turned on and the potential charge of the p-type sense amplifier enable signal SAP flows into the bit line BL. Hence, the voltage of the p-type sense amplifier enable signal SAP node becomes low and is connected to the gate terminal of the first NMOS transistor 13 of the comparator 50A to thereby turn ON the first NMOS transistor 13. Then, the potential at the node N1 is at the logic "low" state and the voltage of the p-type sense amplifier enable signal SAP can be recovered again.
However, under the condition that damage of the voltage on the bit line BL is large generated, the p-type sense amplifier driver 1 of FIG. 2 is slightly turned 0N and the voltage of the p-type sense amplifier enable signal SAP node drops due to the bit line BL. However, the sense amplifier control circuit of FIG. 2 does not have a structure capable of achieving the rapid recovery of the p-type sense amplifier enable signal SAP, so that a slow recovery capability of the bit line BL and the bit line BL appears. As a result, this causes the restoring time of the memory cell to be reduced, thus making it impossible to completely recover cell data. Specially, this seriously appears upon data transmission of a dual port memory.